The present invention relates to an improvement in the process and apparatus for controlling the separation of air to obtain oxygen, argon, and nitrogen products and, more particularly, for controlling the composition of the feedstream to a crude argon column. Such a feedstream composition is a crucial parameter in the crude argon production process. The nitrogen content of the feedstream which can change rapidly affects the stability of the process.
The crude argon column feedstream is customarily taken from the low pressure column of an Air Separation Unit (ASU) at a point which is in the vicinity of the peak argon concentration. This region is termed the argon band. A typical feedstream composition is about 100 ppm nitrogen and 10% argon with the balance of oxygen. However, actual compositions depend on the particular plant design and how it is being operated. Maintaining the correct level of nitrogen in the feedstream to the crude argon column is however very important for the following reasons:
(1) If the nitrogen concentration is too high, the nitrogen pressure in the top product of the crude argon column will have a detrimental effect on the heat transfer duty of the argon condensor which in turn will negatively affect the flow of gas up the column. At some stage the gas flow will drop below that needed to properly support the liquid on the trays and the liquid will flow out of the trays towards the bottom of the column. This is known as dumping the column. Dumping the column leads to a loss in argon production and a serious dump could require about a day for recovery. Moreover, the liquid oxygen product in the low pressure column of the ASU can become contaminated with dumped argon liquid.
(2) If the nitrogen concentration is too low, this implies that the argon band is relatively high in the low pressure column of the ASU and more argon is being vented with the waste nitrogen. The result is that the argon recovery is not maximized. As a minimum liability, more energy is being expended for the recovery of argon than is necessary.
For a given situation, there is an optimum nitrogen concentration in the feedstream to the crude argon column corresponding to maximum argon recovery. However, since the argon band happens to be located in the tail of a steep nitrogen concentration gradient, the amount of nitrogen in the feedstream can vary rapidly from the tens of ppm to thousands of ppm in response to relatively small changes in the plant, while on a percentage basis the corresponding variations in argon concentration, though important with respect to the argon recovery rate, may be comparatively small.
If the feedstream nitrogen levels could be maintained closer to the optimum level, the average argon production rates could be significantly increased. An improvement in argon production of about ten percent is possible using the present invention.
The composition of the feedstream to the crude argon column in a typical air separation plant is a sidestream going from the low pressure column of the ASU to the crude argon column. This composition may be affected and controlled by oxygen product withdrawal rates. For instance, if the oxygen withdrawal rate is increased, then the argon band will be shifted down the column resulting in an increase in the argon and nitrogen concentration in the feedstream to the crude argon column. The inverse situation will occur if the oxygen withdrawal rate is decreased. Since the former situation is less desirable than the latter situation, the tendency is to operate a plant conservatively, or in other words, sufficiently far away from the dumping condition so that the approach of a dumping condition can be noticed and corrected in time to avoid a significant dump. The approach of a dumping condition may be signaled by an increase in the nitrogen in the crude argon product stream of the crude argon column and a decline in the pressure differential across the trays in the crude argon distillation column. Since the process is typically controlled manually, considerable skill and experience is required to achieve consistently high rates of argon production.
Prior art control techniques have been unsatisfactory. For example, as one basis of control, the argon and oxygen concentratios in the crude argon product were monitored, whereupon the amount of nitrogen (which is the balance) was given by the difference. The nitrogen concentration at this point is about one or two percent, whereas the nitrogen concentration in the feed gas to the crude argon column is typically in the hundreds of parts per million. This difference is due to the fact that the nitrogen tends to be concentrated along with the argon. However, monitoring the process by monitoring the nitrogen in the product is like measuring the accumulated effect of a control error rather than the control error itself.
Other past techniques for indirectly controlling the nitrogen concentration in the sidearm feedstream to the crude argon column include measuring a change in pressure or by monitoring temperature levels on certain trays and adjusting production rates of argon withdrawn from the auxillary rectification tower (the crude argon column), such as disclosed in U.S. Pat. No. 2,934,908 to Latimer, or by adjusting the reflux to the primary rectification unit (the low pressure column of the ASU), similarly in response to temperature levels, such as disclosed in U.S. Pat. No. 2,934,907 to Scofield. Such adjustments to process conditions, however, suffer from either insensitivities or delays in response to sensed conditions inherent in the operation of the rectification process.
Another previous control technique involved measuring the percent of oxygen in the feedstream to the crude argon column, thereby inferring the nitrogen concentration. When the oxygen decreased, it was generally inferred that the nitrogen had increased. However, the nitrogen versus argon concentration could not be determined. The system was controlled normally by the oxygen withdrawal rate. However, this control scheme presented certain drawbacks. In particular, if argon was building up, the correct response would be to draw more argon out of the crude argon column. Instead, the analyzer might cut off the argon product, by incorrectly inferring build up of nitrogen. Furthermore, this control was not sufficiently sensitive and too slow. As a result of the state of the art, the process was run conservatively, thereby not optimizing argon production.